Centrifugal reactor with residence time control

ABSTRACT

A method and apparatus for chemical processing. The apparatus comprises an axis about which a plurality of supporting elements are rotated to provide an enhanced gravitational field. Each supporting element is inclined to the axis and each other such that one or more flowable and reactive components can pass from one supporting element to the next in a closely controlled chemical reaction sequence. The one or more supporting elements comprise means, such as holes or notches, for disrupting the flow of the reactive components to either promote mixing or enable free gas or vapour passage.

This invention relates to a fluid control device for chemicalprocessing. The invention has application to chemical reactors and inparticular to a method and apparatus employing thin film technology.

All chemical reactions involve changing the molecules involved. Reactivecomponents can present themselves as liquids, solids, gases or vapours.A perfect reactor would provide the ideal pattern of contacting for anoptimal time to achieve the desired outcome.

Most reactions in the laboratory and in batch production involvingliquids utilise stirred or agitated vessels to provide the necessarycontacting between components. The liquid phase may hold a number of thereacting species in solution and molecular contact. Unless impeded bymicelles, this contact is normally intimate and mainly impeded byviscosity.

One or more of the components in the liquid phase may be immiscible andmay take the form of an emulsion where molecules in the suspended phasescontact the components in the continuous phase and any other immisciblecomponents by diffusion. Similarly gases may be present in the form ofbubbles.

In most cases the more severe the agitation the faster the reactionproceeds until it is only limited by the kinetics of the chemistry. Thereaction proceeds with time until it is deemed complete. Time proceedsat the same rate throughout the tank and so as long as the agitationlevel is evenly spread at a more than necessary level throughout thetank, the reactions are evenly spread through the reaction vessel withreaction progressing evenly throughout the vessel with time.

Immiscible components are those in which the molecule population densityof the other components is substantially lower than could be achievedwere the components fully soluble in each other. Thus, solids, gases andvapours may be described as immiscible components as they aresubstantially immiscible with the liquid components.

It is recognised that often process outcomes depend on diffusion betweenand into these media and so mixing and diffusion path lengths areimportant aspects of performance.

Difficulty is encountered with traditional batch reaction equipmentbecause the reactions occur at a molecular scale and the mixing is on amuch greater scale. On scale-up from the laboratory it is increasinglydifficult to maintain the ideal reaction conditions throughout thereaction space.

For example, transferring heat into and away from energetic reactionscan prove difficult. Bulk mixing is generally easy but mixing at amicroscopic level is difficult to achieve and discontinuous phases tendto aggregate so bubbles become larger and liquids of different densityseparate.

All of these factors, and more, severely limit the effectiveness ofreactors and so yields and adverse by-products can be far from ideal.Molecular mobility can be enhanced by lowering viscosity and so manyreactions are often conducted utilising solvent systems to lower theviscosity and improve miscibility.

Much of chemical engineering effort has been directed to counteringthese effects on scale-up and chemists, being aware of the difficultiesof scale-up, normally endeavour to provide process sequences whichminimise these known difficulties. Solvents in particular are utilisedto bring reactive components together and to separate out products bothdesirable and undesirable. A less scale sensitive reaction sequence canbe achieved by deliberately slowing the reactions by diluting thecomponents, limiting the feed rates and operating at low temperatures.

It is well known that thin films can be utilised to provide an intimatecontact with a heat transfer or catalysed surfaces and that disturbingthe film can assist mixing and refresh the surface to promote masstransfer with a gas or vapour.

Such films can be created for example by wiping and spreading and thereare many generic devices to achieve this outcome. Providing a packedstructure that permits liquid components to drip from one packingcomponent to the next are known to improve the effectiveness ofliquid/gas mass transfer.

The present invention has been made from a consideration of theforegoing and is more fully described hereinafter with reference toFIGS. 1 to 16 of the accompanying drawings that depict various featuresaccording to different aspects of the invention that may be employedseparately or in combination as desired.

The liquids referred to in this application may be single components,multiple components in solution, immiscible combinations of liquids,liquids containing non-liquid components, and solids that are able toflow like liquids, such as spherical beads or powders or solidssuspended on or in liquids. The liquids may carry with them homogeneousor heterogeneous catalysts including phase transfer catalysts.

According to one aspect, the present invention uses means such asperforations or edge features, for example notches, in an inclinedsupporting plane carrying liquid in an enhanced gravitational field toproduce fibrils of liquid which can be utilised to promote mixing,enable free gas and vapour passage

The perforations and/or edge features (see FIGS. 2 and 3) can be used toprovide different residence times for the molecules flowing through theequipment to provide beneficial material properties.

The perforations and/or edge features may be used on inclined planes toprovide for controlled mixing of a number of liquids to provide aclosely controlled chemical reaction sequence.

According to another aspect, the present invention utilisesgravitational forces, normally induced by centripetal acceleration, tofacilitate the generation of thin films.

The liquid when unsupported falls freely in the field until itencounters an accelerating element (see FIG. 1). If that element islarge enough and inclined to the acceleration field the liquid willspread and form a thin film the depth of which can generally be definedby Nusselt's equation for flow down an inclined plane.

According to yet another aspect, the present invention uses fibrilsinduced by flowing the film through holes, cuts and notches (see FIGS. 2& 3) that can be utilised to provide for controlled residence timedistribution, diffusion, mixing and reactions.

Even when utilising substantially immiscible components, a reactiveenvironment is provided in which the components can be brought intointimate contact with each other in a flowing stream or streams. Thetime taken for the reaction to proceed also involves a displacementthrough the reactor.

According to a still further aspect, the present invention uses acombination of immiscible liquids flowing on an inclined plane which maybe subjected to enhanced gravitational fields to produce a flow close toplug flow at Reynolds numbers which are very small where normally theflow profile of a single component would normally be a parabolic laminarflow profile with zero velocity relative to the plane as its minimum.

Embodiments of the invention permit fluids to achieve much thinner filmsthan could be achieved with the same equipment and with the same massflow rate with any one of the fluids flowing on its own.

Chemical reactants including liquids, solids and vapours can be inducedto pass readily between the films enhanced by short path lengths andgravitational forces where there is a density difference.

Temperature control can be facilitated by this vapour transport byevaporation and condensation

Embodiments of the invention also describe how the streams may beselectively separated and re-combined by utilising troughs in the planesto gravitationally separate out components that may be re-introducedlater.

As well as conventional reactions, the use of more than one immiscibleliquid provides the opportunity to utilise one or more liquid electrodesto create an electrochemical cell which may have plate separations of afew microns.

Ideally, some embodiments of the invention seek to achieve a flow fieldwhere the flow is entirely one dimensional (plug flow) with the distancethrough the reactor being a function of the time that the reactants havebeen present within the reactor irrespective of the position of themolecules in the film cross-section. The more time sensitive thereaction, for example competitive reactions where the products reactwith themselves and/or the feedstocks, the more critical it is that plugflow should be achieved.

It is normally assumed that the best characteristic that can be obtainedfor a flow reactor is limited by the velocity profile, with a laminarflow profile being a typical representation of the residence timedistribution. This is normal for a reactor operating under highacceleration where one normally changes the film shear, film thicknessand residence time by modifying reactor rotational speed and the streammassflows. However, the present invention provides other methods tocontrol these which can be used to improve the outcomes of processes.

More particularly, some embodiments utilise combinations of immisciblefilms in acceleration fields which may be substantially greater thanthat of gravity resulting in a technique that permits the residence timeprofile of one or more of the liquid components to undergo substantiallyplug flow (see FIGS. 4,5 & 6) in a film with a Reynolds number wellbelow 100 and more often well below 1.

The films can be maintained with a thickness of between 1 and 1000microns in many cases. The film profiles are affected by the fluiddensity, mass flow and viscosity as well as the geometry andacceleration.

Fluids can be chosen to achieve a specific profile, for example if a lowviscosity liquid flows down an inclined plane together with a higherviscosity, lower density, immiscible material e.g. liquid metal andpolymer, then the result is that the low density material will form aseparate layer above the high density material.

The liquid next to the supporting plane will be substantially stationarywith respect to it but the surface away from the supporting plane willbe relatively free flowing in the direction of the slope. For any givenacceleration the thicker the first film and the lower its viscosity thefaster its surface will flow.

The higher viscosity material which is subjected to the sameacceleration will move much more readily than it would have done were itcontacting a solid wall and hence be much thinner for a given mass flowand have a relatively small difference in velocity between the bottomand the top of the film.

Thus, in this circumstance not only is the flow close to a plug flow forthe film away from the surface but the film thickness for a given flowis much thinner than it would otherwise be. This reduces residence time,the residence time distribution and diffusion path lengths all of whichmay be beneficial.

In the case where the reaction is significantly exothermic orendothermic, it is possible to utilise evaporation or condensation fromor to one or more of the liquid streams (see FIGS. 7 & 8). This may bedone as well as utilising the heat capacity of the flowing liquids, heatexchange to the supporting planes, energy exchange by infra-red, radio,ultra-violet, inductive, light and vibration, and to the gas or vapourflowing above the free surface.

It is also possible to introduce an immiscible stream specificallyintended to provide thermal control by evaporation or condensation.Where vapour exchange is occurring on the free surface, temperaturecontrol can be extremely tight as vapour exchange can be at very highrates and the pressure closely controlled.

Where a volatile film is located in one of the lower streams, it mayevaporate by nucleate boiling but more commonly will undergo evaporationand hence flash cooling when it falls from one plane to the next. Theheat of reaction in the liquid on the supporting plane may be absorbedas sensible heat in this and all the other components as they flow downthe plane and may be released in part or in whole in vapour exchange,with a consequential temperature change, as the components fall from oneplane to the next.

Where the reaction is exothermic or endothermic the density changeproduced by the heat of reaction may be utilised to move the reactedcomponents away from the reaction site (see FIG. 9).

Given the low Reynolds number encountered when such films are flowingdown inclined planes it is difficult to induce effective mixing byperturbation. Under the effects of high acceleration fields, when thefluid leaves the surface it tends to form a thin sheet, as the surfaceaccelerates away from it, with a thickness substantially less than thatof the film on the supporting plane.

We have found however that, if the liquid is permitted to flow throughholes in the supporting plane or over notches in the edge it can beinduced to form fibrils rather than sheets (see FIGS. 1, 2 & 3). As thedistance from its point of origin increases the fibril, of a non-fibreforming material, will ultimately coalesce into droplets of a similarscale to that of the fibril.

This fibre forming effect substantially changes the flow profile of theflowing film leaving the plane, increases the surface to volume ratio ofthe liquid and so ensures both good mixing and surface renewal whenpassing from one supporting plane to the next.

The mixing effectiveness can be further enhanced by inducing a velocitydifference between the one plane and the next (See FIG. 10). This can befurther enhanced by features on the receiving plane impacting on thefalling fibrils and droplets causing them to break up.

In equipment generating high acceleration fields, with significant gasvelocities, to deliberately form fibrils has the advantage of notimpeding gas flow as much as sheets of liquid. This can be critical asthere can be a significant pressure build up from one side of the sheetto the other which can force the film to be displaced or broken uprandomly or even to substantially impede the gas flow. This lattereffect is particularly critical with low pressure systems such as areroutinely required in devolatilisation devices or polycondensationreactors.

Gases and vapours can be induced to flow radially inwards if so desiredby the use of appropriate seals. These may be liquid seals. Such sealscan also be utilised to provide for different chambers in the reactorwhich may contain gases or vapours of different compositions and/orpressures.

Because the films are substantially laminar there is excellentmicro-mixing along the shear plane which promotes contact betweenspecies in that plane and hence reaction. Mixing normal to the shearplanes is achieved by diffusion and the short path length provided bythe thin films promotes this.

The novel techniques described above provide for ways of thinning thefilms and provide a controlled shear in the reacting film. We havediscovered that it is advantageous to repeatedly break the film intofibrils and then re-form them into a rectilinear flow field on asubsequent surface. Increasing the frequency of this action increasesthe mixing.

This effect is particularly important when solubility or diffusion ratesare limited, for example for viscous polymers requiring to be strippedof a volatile component. Also the dispersion of fine solids in theliquid stream is enhanced as the buoyancy forces due to densitydifferences are substantially eliminated in the free-falling fibrils.

The desired frequency of these jumps from one surface to another may bechosen according to the requirements of the process. For example, if aprocess is dominated by mixing solid particles into a liquid streamthen, in general, the more frequent the interruptions should be. Wherethere is little requirement for frequent surface renewal and generalmixing, then the more the surface can beneficially be continuous.

Cascading supporting planes can be in the form of a series of steppedjumps in the plane such as one would see in a series of waterfalls. Theycan also be nested, coherent or random separations between planes. Inthe limit, they can be of a similar scale to the fibrils and bepositioned to simply break the fall of the fibrils and re-form themusing an ordered or random structure.

In some circumstances, there may be a requirement for a reactor not toshow a plug flow characteristic. For example, in polymerisationreactions, such as ring opening polymerisation, as long as unconnectedrings are available a catalysed polymer chain will keep growing. Thus atrue plug flow reactor would produce a very narrow molecular weightdistribution. If a wider molecular weight spread is required or, in thelimit, a particular population of different sized molecules, then thereis a requirement to provide different residence times for the populationof molecules. An ideal reactor would achieve this residence timedistribution by design.

This can be readily achieved with the present invention by utilising areactor format where some of the liquid flow falls onto a plane below itin the acceleration field earlier than other parts of the stream.

Thus, for example, inclined planes may be nested (see FIG. 11) such thatthe first plane is sloped one way and the second plane is sloped theother way under the first plane with a hole made in the first plane partway down the slope. Alternatively or additionally, part of the firstplane may have a shorter path length. As a result, some liquid will exitonto the second plane sooner than other parts of the flow stream thusavoiding the requirement to flow the full length of the first plane andfrom the start of the second plane.

The same effect could be achieved by having some of the liquid on planesin higher or lower acceleration fields, of different lengths, number ofplanes, of different slopes or any combination of these.

Another embodiment uses blading in a spiral form (see FIG. 13) whichprovides a continuous slope into which holes, notches or features may becut to achieve residence time control.

The operating principles above described can be readily employed in arotating reactor operating at an acceleration (acceleration due torotation=speed of rotation²×radius) well in excess of the earth'sgravitational field with the liquid stream supporting planes of thereactor inclined with respect to the resulting acceleration field.

The planes may be in the form of cones, truncated cones (see FIG. 12),spirals or coherent or random packings. The inclination angle of theplanes may vary from being in line with the acceleration field to normalto with constant angles, discretely changing angles or curved forms. Itis possible for the flow to continue with the acceleration fieldreversed utilising surface tension to hold the fluid to the surface.This situation is encountered for example where the surface of a tube isused with the axis of the tube normal to the acceleration field. Wherethe angle approaches that of the acceleration field, the flow may changefrom laminar to wavy. This may beneficially enhance the mixing andsurface renewal effects.

Further to this residence time control, it is possible create very thincoherent films of different density immiscible liquids and solids. Thesemay be thin enough to substantially overcome the diffusion barrierswhich would otherwise substantially limit the rate of chemical reaction.

Whenever there is a density difference between materials an enhancedacceleration field will increase buoyancy and so tend to separate thosecomponents. Thus, components that normally would not separate readily,and emulsions, will tend re-form into coherent thin films.

Immiscible reaction products, whether solid or immiscible liquid, can beseparated readily from the reactants if there is a density differencebetween them and the reactants. They may be carried preferentially intoother streams to be dissolved or be swept along with a stream or becarried between two liquid streams of greater and lesser density or tothe surface or the supporting plane.

The highest density component can be separated from the others in thestream by flowing it into a well (see FIG. 14). If a sequence of suchwells is presented to the flow stream then all of the different densitystreams can be extracted and removed or re-formed in any combination andsequence. The outflow from the well can be achieved via a weir as shownor by any other means such as a float valve or a pressure operatedvalve.

The use of nested inclined planes also facilitates the introduction ofnew components to the flow stream in a controlled way. For example, ifit is desired to add a new component slowly to the stream, it can beselected to flow through a hole(s), notche(s) or edge(s) in a first feedplane positioned such that the flow encounters a flow on a second planeat different points down the incline. If these two planes are conical inform and rotating at different speeds, then each feed flow would bedelivered evenly parallel to the direction of flow on the second cone.

FIG. 15 shows two nested cones where a liquid can be introduced in Zone1. Another liquid may be added in Zone 2 to exit at a single point alongthe cone if holes are located by an annular barrier. Zone 3 shows liquidadded continuously over a length of the zone. Zones 4 and 5 show liquidadded in a controlled sequence from two or more discrete streams. Zone 6and 7 show liquid added firstly over a length of the cone from Zone 6and subsequently from Zone 7.

One advantage of such a device is that the film distributioncharacteristics of the cone can be utilised to evenly distribute therelatively stable feed materials before they encounter the instabilityinherent within a reactive environment. Feed materials may also be fedinto the system and removed utilising troughs and tubes.

If such a feed cone is segmented then number of different materials canbe added in a controlled sequence. Some of these materials may free flowor be pumped from the separated process streams or be new materials.Pumping may utilise the peristaltic pressure available from the fluiditself and/or other means. The use of pumping allows for the possibilityof one more of the liquid or solid components being moved radiallyinwards in stages thus allowing for a stepwise counter-flow of liquidand solid components against the acceleration field.

The flow on the feed cone itself may be part of the reactive sequencesuch as activating feed materials immediately prior to joining the mainreaction sequence. This may require a further feed cone inboard of thefirst one and so on. It is possible for there to be a significant numberof feed systems located radially inboard of each other, axiallydisplaced or close coupled as separate devices.

It is also possible to enhance reaction utilising heterogeneous orhomogenous catalysts fixed or flowing with the streams. It is alsopossible to utilise ultra-violet light or other energising fieldradiating through the films or fibrils. Where critically timed exposureis required the radiation source can be incorporated into the supportingplane or masked by the addition or removal of an opaque film.

It is also possible for electric fields or currents to be applied to thestreams to promote electrochemical reaction (see FIG. 16). It may benecessary in such devices to have insulating solid materials andnon-conducting fluids introduced to facilitate this. Where significantcurrents may flow or to facilitate application of the electrical fields,electrodes may be introduced into one or more troughs in the inclinedplane to facilitate effective coupling to a liquid stream. Thusconducting liquid streams may in themselves form electrodes which may ormay not produce reactive species. It is possible to induce the requiredcurrents by inductive coupling utilising a magnetic field or bycapacitive coupling through the films.

According to another aspect, the present invention provides a chemicalreactor embodying any one or more of the novel features described hereinseparately or in combination.

Examples of two reactors embodying the invention are illustrated inFIGS. 17 and 18 where FIG. 17 shows a co-rotating laboratory reactorwith evenly spaced holes (not shown) along the edge of each blade andFIG. 18 shows a co-/counter-rotating reactor within a pressurecontrolled shell.

As will now be appreciated from the foregoing description of exemplaryembodiments, the present invention utilises the characteristics of flowin an enhanced acceleration field to provide a reaction environment ableto be tuned to be close to optimal that can be employed in a number ofchemical systems.

It is a general requirement for the reactants entering a reaction systemto be sufficiently stable to endure the time required to complete acontrolled reaction. The reactor is deliberately intended to reduce thefeedstock stability by catalytic or chemical means and convert thereactants into desired products with high effectiveness. An idealreactor would therefore take in stable reactants, promote the desiredinstability and re-stabilise conditions before exit. Ideally a series ofreactive steps could be taken in a single device. The spinning conereactor of the present invention may be constructed to achieve thisobjective.

As will be apparent, the present invention provides a method andapparatus for effecting chemical reactions employing thin filmtechnology having numerous benefits and advantages that can be used in aspinning cone reactor as described.

For example, by using notches and other edge features, and perforationsin an inclined supporting plane carrying liquid in an enhancedgravitational field, fibrils of liquid are produced which can beutilised to promote mixing, enable free gas and vapour passage.Furthermore, these perforations and edge features can be used to providedifferent residence times for the molecules flowing through theequipment to provide beneficial material properties. Moreover, theseperforations and edge features on inclined planes can be used to providefor controlled mixing of a number of liquids to provide a closelycontrolled chemical reaction sequence.

It will be understood that the exemplary embodiments described hereinare intended to illustrate the diverse range and application of theinvention and that features of the embodiments may be employedseparately or in combination with any other features of the same ordifferent embodiments.

Moreover, while the exemplary embodiments described and illustrated arebelieved to represent the best means currently known to the applicant,it will be understood that the invention is not limited thereto and thatvarious modifications and improvements can be made within the spirit andscope of the invention as generally described herein.

1. An apparatus for chemical processing comprising an axis about which aplurality of supporting elements are rotated to provide an enhancedgravitational field, each supporting element being inclined to the axisand one another such that one or more flowable and reactive componentscan pass from one supporting element to the next in a closely controlledchemical reaction sequence, characterised in that the one or moresupporting elements comprise means for either controlling the residencetime of the reactive components flowing through the apparatus and/orcontrolling mixing of the reactive components as the reactive componentspass from one supporting element to the next.
 2. An apparatus as claimedin claim 1 wherein the means for either controlling the residence timeof the reactive components flowing through the apparatus and/orcontrolling mixing of the reactive components cause the reactivecomponents to form fibrils or droplets.
 3. An apparatus as claimed inclaim 1 or 2 wherein the means are perforations or edge features.
 4. Anapparatus as claimed in any of the preceding claims wherein thesupporting elements are nested such that a first plane of a firstsupporting element is nested one way and a second plane of a secondsupporting element is sloped the other way under the first plane.
 5. Anapparatus as claimed in claim 4 wherein the planes are in the form ofcones, truncated cones, spirals or coherent packings.
 6. A process forcontrolling residence time or mixing between one or more flowable andreactive components in an apparatus for chemical processing comprising aplurality of supporting elements, characterised in that a first reactivecomponent is caused to leave one supporting element for another as afibril or droplet.
 7. A process as claimed in claim 6 wherein thereactive components are caused to leave one supporting element foranother repeatedly along the closely controlled chemical reactionsequence.
 8. A process as claimed in claim 6 or 7 wherein the reactivecomponents travel along the supporting elements as films.
 9. A processas claimed in claim 8 wherein at least one film exhibits plug flowcharacteristics.
 10. A process as claimed in any of claims 6-9 wherein areactant falls onto a subsequent support element in an acceleratingfield earlier than in other parts of the closely controlled chemicalreaction sequence.
 11. A process for reacting at least two chemicalreactants on an apparatus for chemical processing comprising an axisabout which a plurality of supporting elements are rotated to provide anenhanced gravitational field each supporting element being inclined tothe axis and one another such that one or more flowable and reactivecomponents can pass from one supporting element to the next in a closelycontrolled chemical reaction sequence, characterised in that thesupporting elements are arranged or modified or the operating conditionsare modified such that one reactive component is able to exit onto aavoiding the requirement to flow the full length of the first supportingelement and from the start of the second support element.
 12. A processfor reacting at least two chemical reactants on an apparatus forchemical processing comprising an axis about which a plurality ofsupporting elements are rotated to provide an enhanced gravitationalfield each supporting element being inclined to the axis and one anothersuch that one or more flowable and reactive components can pass from onesupporting element to the next in a closely controlled chemical reactionsequence, characterised in that the first reactant flows across asubstantially planar support and wherein a second reactant flows overthe first reactant with a flow characteristic close to plug flow.
 13. Aprocess as claimed in claim 12 wherein the first reactant is a carrier.14. A process as claimed in claim 12 or 13 wherein the second reactantis a film with a Reynolds number of below
 100. 15. A process as claimedin claim 14 wherein the film has a thickness of between 1 and 1000microns.
 16. A process as claimed in any of claims 6-15 wherein thereactants are immiscible with one another.